The present invention relates to a multilayer printed circuit board and, more particularly, to a multi-throughhole multilayer printed circuit board employing a material having a low dielectric constant.
In view of the recent demand for high density printed circuit boards, a technique has been developed for forming signal circuit patterns in inner layers, and connecting the inner layer signal circuits to each by partial-layer through holes. To produce signal circuit patterns in inner layers, it is required to limit the impedence of signal lines to a predetermined value. To satisfy this requirement, efforts have been made to design a space between a signal layer and an electric source or ground layer, hereinafter referred to as a source/ground layer, disposed in opposition to the signal layer. Such efforts have been concerned with taking into account the thickness of the insulating layer of each of the copper-plated lamination layer plates forming a multilayer printed circuit board, the thickness of each adhesive sheet layer after formation of the multilayer, a width of each of the signal lines, as well as the dielectric constant of each of the insulating layers.
However, since the material commonly used in the prior art has been limited to glass epoxy or glass polyimide, it is difficult to freely select the dielectric constant of the insulating layer and, since the signal line width is also largely limited by virtue of the print wiring density, it is difficult to freely select the signal line width in high density printed circuit boards. Furthermore, since it is common to form signal circuit patterns on both opposite surfaces of one copper-plated lamination layer in order to facilitate the formation of a multi-through hole multilayer printed circuit board, in practice, two signal layers are generally sandwiched between two source/ground layers and a thickness of the insulating layer sandwiched between the two source/ground layers is controlled so as to obtain a predetermined impedance.
More particularly, where, for example, two signal lines are passed between two through hole pads on one surface of a glass polyimide copper plated lamination plate and are arranged at a pitch of 50 mil (1.27 mm) in order to obtain an impedance of 50.OMEGA., the signal line width will be about 0.1 mm by the wiring density and the thickness of the insulating layer will be about 0.45 mm. If a multi-through hole multilayer printed circuit board having six signal circuit layers is constructed in accordance with the above noted conditions, a multilayer printed circuit board of fourteen layers and of a thickness of 2.0 mm can be obtained by combining copper-plated lamination plates each of which includes an insulating layer having a thickness of 0.11 mm and adhesive sheets each of which have a thickness of 0.05 mm.
Furthermore, in view of the recent demand for an increase in the manufacturing of printed circuit boards, materials for low diaelectric constant copper-plated lamination plates and adhesive sheets have been developed so that a glass butadiene, copper-plated lamination plate having a dielectric constant of 3.5 or less, which dielectric constant is about three-quarters of that of a conventional glass-epoxy or glass polymid copper-plated lamination plate, has recently been put into practical use. This is based on the fact that the propagating speed of an electric signal is determined on the basis of the dielectric constant and magnetic permeability of the material through which the electric signal propagates.
To freely use the copper-plated lamination plates and adhesive sheets, measures are required to particularly counter an increase in dimensional variations of the copper-plated lamination plate, as well as measures to avoid the occurrence of a void adhesive substance phenomenon, that is, an existence of a residual air layer in the adhesive layer, in the multilayer bonding process. Additionally, further measures are required to preclude the occurrence of wrinkles and/or the breakage of the copper-laminated plates and adhesive sheets during the handling thereof.
The aim underlying the present invention essentially resides in providing a multilayer printed circuit board and a method for producing the same in which a material having a low dielectric constant can be used without increasing the overall processing or total costs in production of the circuit board. In accordance with advantageous features of the present invention, it is possible to provide a multilayer printed circuit board and method of producing the same which has substantially the same thickness as conventional multilayer printed circuit boards and which employs an insulating material having a dielectric constant which is lower than the dielectric constant of the insulating materials employed in conventional circuit boards.
In accordance with advantageous features of the present invention, a multi-layer printed circuit board is provided wherein a plurality of copper-plated lamination plates, each of which are provided on opposite sides with a signal layer and a source-ground layer respectively, are stacked in such a manner that the signal layer and the source/ground layer are alternately disposed.
In accordance with further advantageous features of the present invention, an insulating layer between a signal layer and a source/ground layer is constituted by an insulating layer of a copper-plated lamination plate and an adhesive layer.
The above objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, one embodiment in accordance with the present invention .